A vertical-cavity surface-emitting laser (VCSEL) is a laser in which a pair of reflectors and an active layer interposed therebetween form a resonator perpendicular to a substrate. The reflectors used are distributed Bragg reflectors (DBRs) each including several layers, formed by depositing two types of materials with different refractive indexes, having an optical thickness corresponding to a ¼ wavelength. The reduction in loss by increasing the reflectivity of the DBRs is a challenge to increase the laser power of the VCSEL. In order to cope with variations in emission wavelengths during device manufacturing and changes in emission wavelengths due to heating, the reflection band of the DBRs needs to he increased. In usual, the difference in refractive index between two types of materials needs to be large in order to increase the reflectivity and reflection band of the DBRs.
However, in the case of using two different types of materials having different lattice strains, there is a problem. in that crystal defects are induced. In order to cope with this problem, Patent Literature 1 discloses a strain-compensated DBR in which a large number of AlGaN layers having tensile strain for GaN and a large number of InGaN layers having compressive strain for GaN are stacked on a Gall substrate. Furthermore, Patent Literature 1 discloses that crystal quality is enhanced by interposing strain-free GaN layers between AlGaN layers and InGaN layers.